In the USA, the octane numbers are calculated different from Europe.
In numbers that means:
Euro 91 octane = US 86,8 octane
Euro 95 octane = US 90 octane
Euro 98 octane = US 93 octane
Source:
Shell Deutschland Schmierstoff GmbH
Anwendungstechnik Schmierstofftechnik
Suhrenkamp 71-77
22284 Hamburg
Germany